Frequency reuse patterns are cell-based schemes for assigning the frequency channels available within a particular cellular telecommunications system. The most basic unit of any frequency reuse pattern is a cell. Each cell within a frequency reuse pattern is assigned a number of frequency channels. A plurality of cells are then associated together and referred to as a cluster and utilizes all of the frequency channels available to a particular cellular telecommunications system. Groups of clusters are then used to provide a cellular coverage area within the cellular telecommunications system and the frequency channels allocated for one cluster are reused in other clusters. The scheme for recycling or reassigning the frequency channels throughout the serving coverage area is referred to as a reuse plan. The distance between a first cell using a particular frequency channel within a first cluster and a second cell using the same frequency channel within a second cluster is further known as a reuse distance.
The reuse of the same frequency channels by a number of different cells implies that cells may suffer from co-channel interferences. It is therefore desirable for the received strength of the serving carrier (C) within each cell to be higher than the total co-channel interference level (I). As a result, the higher the carrier to interference (C/I) value, the better the speech quality. A higher C/I value is obtained partly by controlling the channel reuse distance. The larger the reuse distance between adjacent cells utilizing the same frequency channels, the lesser the co-channel interferences created between those cells. The C/I ratio is further related to a frequency reuse plan (N/F) where N indicates the number of sites included within a single cluster and F indicates the number of frequency groups. For example, the C/I ratio is directly related to the following equation: DR=(3*F)1/2·R, where: DR is the reuse distance; F is the number of frequency groups; and, R is the radius of a cell. Accordingly, the larger the F value, the greater the reuse distance. However, it is not always desirable to use a larger F value to increase the C/I ratio. Since the total number of available frequency channels (T) is fixed within a particular mobile network, if there are F groups, then each group will contain T/F channels. As a result, a higher number of frequency group (F) would result in a fewer channels per cell and lesser call capacity. Furthermore, in a packet data cellular system (such as in Evolved UTRA) packet transmission takes place over a shared channel where resources are shared by several users. This means a very large number of users may have to compete for the limited resources, reducing peak user bit rate and thereby increasing the packet delay transmission. Increased packet delay is undesirable as it adversely affects the service quality.
For most cellular systems, capacity is not a major issue when the system initially goes into operation. Therefore, in order to achieve a high C/I value and to improve the quality of speech connection, a high frequency reuse plan (N/F), such as 9/27, is initially used. However, as the capacity increases, the cellular telecommunications network has to resort to a lower frequency reuse plan, such as a 7/21 or 4/12, to allocate more frequency channels per cell. Moreover, the success of such systems requires that they are able to offer high peak bitrate and shorter packet transmission delay already during their initial deployment.
In cellular communication there are generally two main modes of operation for duplex transmission on uplink and downlink: Frequency Division Duplex (FDD) and Time Division Duplex (TDD), with their usage typically dependent on the frequency band used. FDD uses paired band where uplink and downlink transmission takes place at different carrier frequencies. Generally, there is also a fixed relation between a frequency band used for uplink and downlink transmission. TDD is used on unpaired bands where common carrier frequency is used for uplink and downlink transmission. One potential advantage with TDD is that frequency bands are more efficiently used. Secondly, the total available radio resources which are defined in terms of uplink and downlink time slots can be dynamically interchanged. This means asymmetric traffic between uplink and downlink can be better handled by adjusting uplink and downlink capacity (i.e., time slots).
Accordingly, there is a need in the art for improved methods, and systems for employing such methods, to optimize frequency reuse in cellular communications systems.